In the broadest sense, our research objectives are to understand biological processes and phenomena in terms of physical mechanisms and to explore the adaptation of physical techniques to problems in biology. The more specific goals and problems to which we are addressing ourselves in this proposal are the following: 1) Structure and Function of Photosynthetic Reaction Centers; To understand the mechanism of energy conversion in a protein-chlorophyll complex solubilized from the membranes of photosynthetic bacteria, our general goal is to characterize and determine the structure-function relation of this complex called the reaction center (RC). Serving as the basic framework of this research is the three dimensional structure of the RC from Rb. sphaeroides R26. Our first aim is to determine the structure of the RC from different strains and mutants of Rb. sphaeroides. Our second aim is to determine the structure of modified RCs; then relate the structural differences to functional changes. To gain further insight into the function of the RC, our third aim is to supplement the traditional static x-ray model by investigating the dynamical properties of the RC through analysis of x-ray data at different temperatures. As a fourth goal, we plan to characterized the interactions between the RC and cytochrome c2. Some of the findings and methodologies are expected to be relevant to other electron transfer processes, most notably the respiratory chain in mitochondria. 2) Electronic Structure of Biomolecules: To compliment the three dimensional structural information, we wish to extend our understanding of the RC to the atomic level. Through a variety of spectroscopic experiments (EPR, ENDOR, and linear dichroism), we plan to characterize the electronic structure of the non heme iron, quinones, and other cofactors. 3) Crystallization of Membrane Proteins: The first general goal is to understand the mechanisms which lead to the formation of single crystals in complex, multiphase solutions. The long term objective is to arrive at a systematic procedure to obtain large, high quality, crystals for x-ray diffraction study. The second goal is to use the empirical methodologies that have been proven successful in growing crystals of the RC to crystallize other membrane proteins of bacterial systems as well as other biological systems, such as plants.